The exhaust gas emitted from an internal combustion engine is a heterogeneous mixture that contains gaseous emissions such as carbon monoxide (“CO”), unburned hydrocarbons (“HC”) and oxides of nitrogen (“NOx”) as well as condensed phase materials (liquids and solids) that constitute particulate matter. Catalyst compositions, typically disposed on catalyst supports or substrates that are disposed within the exhaust system of an internal combustion engine are provided to convert certain or all of these exhaust gas constituents into non-regulated exhaust gas components. For example, exhaust systems for internal combustion engines may include one or more of a precious metal containing oxidation catalyst (“OC”) device for the reduction of CO and excess HC, a selective catalyst reduction catalyst (“SCR”) device for the reduction of NOx, an adsorber to capture and store NOx until the SCR device reaches an operational temperature and a particulate filter (“PF”) device for the removal of particulate matter from the engine exhaust gas.
As indicated, a technology that has been developed to reduce the levels of NOx emissions in lean-burn engines (ex. diesel and gasoline fueled engines) that burn fuel in excess oxygen includes a selective catalytic reduction (“SCR”) device. The SCR catalyst composition may contain a zeolite and one or more base metal components such as iron (“Fe”), cobalt (“Co”), copper (“Cu”) or vanadium (“V”) which can operate efficiently to convert NOx constituents in the exhaust gas in the presence of a reductant such as ammonia (‘NH3”). Although the use of a catalyst aides in the reduction of activation energy that is required for the SCR device to reduce NOx, the ever increasing efficiency of diesel and other lean burn engines results in cooler exhaust temperatures when moderately operated and following engine cold start-up. Such cooler operating temperatures delay the operational start-up of the SCR device, which must reach a minimum operating temperature to effectively reduce NOx. Typically, an SCR may not reach appropriate operating temperatures until several minutes after the engine is started.